In the prior art, a semiconductor heat treating process is generally carried out in a furnace of the following design in order to prevent any contamination from the ambient air, heater or the like. As shown in FIG. 4, a furnace core tube 21 of high-purity quartz or the like which is provided with an opening 22 at one end and a gas inlet conduit 23 at the other end is disposed inside a cylindrical heater 24 which is previously installed in the furnace. A boat 26 of high-purity quartz or the like having semiconductor substrates 25 rested thereon is moved into the furnace core tube 21 through the opening 22 and set in place. A lid 27 of high-purity quartz or the like is closed to establish a substantially tight seal. High-purity gas such as nitrogen or argon is fed from the gas inlet conduit 23 whereas the gas is discharged out of the furnace through small gaps between the core tube 21 and the lid 27. While the in-furnace atmosphere is kept clean in this way, heat treatment for dopant diffusion or oxidation is carried out.
FIGS. 7A-7C illustrate one exemplary heat-treatment flow using a heat-treatment furnace including such a core tube and FIG. 10 illustrates one exemplary time sequence of the heat treatment. The prior art heat treating process is described by referring to these figures.    (1) A predetermined number of semiconductor substrates 25 are rested on the boat 26, which is held on standby at a boat station 30. See FIG. 7A and FIG. 10 (i).    (2) The lid 27 of the core tube 21 is opened. The boat 26 having semiconductor substrates 25 rested thereon is forcedly moved from the boat station 30 to a predetermined position at the center of the furnace, using a rod of high-purity quartz or the like (not shown). See FIG. 7A and FIG. 10 (ii) and (iii).    (3) The lid 27 is closed. The semiconductor substrates 25 are heat treated according to the predetermined thermal profile. See FIG. 7B and FIG. 10 (iv).    (4) After the heat treatment, the lid 27 is opened. The boat 26 having semiconductor substrates 25 rested thereon is forcedly moved from the predetermined position in the furnace to the boat station 30, using a rod or the like, and cooled there. See FIG. 7C and FIG. 10 (v) to (vii).
For the duration from insertion to removal of the boat, high-purity gas such as nitrogen may be continuously fed from the gas inlet conduit 23 to keep cleanness in the furnace.
In this process, after the boat is taken out of the furnace, the semiconductor substrates must be cooled at the boat station until a handleable temperature is reached. When it is desired to continuously carry out a predetermined heat treatment, the system remains unready for insertion of a next batch, introducing a standby time between consecutive heat treatment batches.
Besides the furnace core tube mentioned above, the structure of a heat treatment furnace for use in the semiconductor substrate heat treating process is proposed, for example, in JP-A H05-102054 (Patent Document 1, Sony Corp., diffusion furnace). The heat-treatment furnace is described therein as comprising a furnace core tube which is open at one end, a shutter for operatively closing the open end, and a partition disposed in the core tube and inside the open end so as to define a gap between the partition and the inner wall of the core tube, the shutter being provided with a vent. This design allegedly eliminates any adverse impact by entry of ambient air.